Our power calculation focused on our primary outcome, no contraception use at last sex in the previous 6 months, which we anticipate being of 25% prevalence among our comparison group at follow-up 2, informed by the previous Ripple trial. Assuming this prevalence, in order to detect a halving of risk (in the range of effects reported by for example the Safer Choices and CAS-Carrera studies) with 80% power and 95% precision would require a sample of 180 per arm.